The present invention relates to a power supply system through an optical fiber cable, in particular, relates to such a system in which a telephone subscriber is connected to an exchange station through an optical fiber cable, and the energy for operating the telephone set of that subscriber is supplied from the exchange station through that optical fiber cable.
In a conventional telephone communication system, telephone subscribers are connected to an exchange station through a metallic wire or a metallic cable, and the energy for operating a telephone set at each subscriber is supplied from an exchange station through that metallic wire. The energy for operating a telephone set is usually DC (direct current) current, but is not AC (alternate current) current.
There is a possibility that telephone subscribers can be connected to an exchange station through an optical fiber cable, instead of a metallic cable, in the future, since an optical fiber cable is excellently suited in that (1) the manufacturing cost of an optical fiber cable is expected to be lowered since the material of an optical fiber cable is cheap compared with that of a metallic cable, (2) an optical fiber cable can provide a high speed pulse communication, and (3) an optical fiber cable is light in weight, and small in size.
When an optical fiber cable is utilized to connect telephone subscribers to an exchange station, the problem is how to supply power to subscribers from a central station. That is to say, the station can not supply electrical power through an optical fiber cable, which is not conductive. A commercial electrical power supply provided at each subscriber is not convenient in case of the interruption of the power supply service. A battery installed in each subscriber is also inconvenient for the maintenance of a battery. An interstitial cable for the power supply along a communication optical fiber cable is not desirable since two kinds of cables must be installed and the investment for cables will be doubled.
The most promising proposal for the power supply in case of an optical fiber cable, is to transmit an optical energy from an exchange station through the optical fiber cable and to convert the optical energy to an electrical energy at the subscriber's side. That proposal is shown in "New devices used to power experimental phone over a glass fiber" IEEE Communications Magazine, March 1979, pp. 33-34.
FIG. 1 is the block diagram of that prior power supply system from an exchange station to a subscriber proposed by said article.
In FIG. 1, the reference numeral 1 is a terminal device in a subscriber or a telephone set, 2 is an exchange station, and 3 is an optical fiber cable connected between the station and the subscriber. The station 2 operates of course as an exchange station. The terminal device 1 has an optical-electrical converter 11, a DC booster 12 which raises the DC (direct current) voltage obtained at the output of the converter to the desired voltage, 13 is a demodulator which demodulates the output of the converter 11 to the form suitable to a receiver 14. The reference numeral 15 is a microphone, 16 is a modulator which modulates the voice signal from the microphone 15 to the form suitable to the transmission in an optical fiber cable, 17 is an electrical-optical converter for converting the electrical signal from the modulator 16 to the optical signal, which is transmitted to the station 2 through the optical fiber cable 3. Also, the station 2 has an electrical-optical converter 21, an optical-electrical converter 22, and a power pulse generator 23 which generates pulse signals as a power supply. The output of the converter 22 and the input of the converter 21 are connected to a telephone exchange system through a conventional modulator/demodulator.
In FIG. 1, the power pulse generator 23 installed in the station 2 generates the periodic pulses which have the energy during the hatched duration (x) in FIG. 2, and those pulses are converted to an optical form by the converter 21. The converted optical pulses are transmitted to the subscriber 1 through the optical fiber cable. The optical-electrical converter 11 in the subscriber 1 receives those optical pulses and converts them to an electrical form, which is applied to the DC booster 12. The DC booster 12 rises the DC voltage to a desired value, and stabilizes the same, which is supplied to other circuits in the subscriber 1.
The station 2 transmits a voice signal to the subscriber 1 in the form of a pulse modulation signal (pulse which modulation signal, or pulse amplitude modulation signal). That pulse signal is inserted in the duration (z) of FIG. 2. That pulse signal is converted by the converter 11 from the optical form to the electrical form, and the converted pulse signal is applied to the demodulator 13, which demodulates the signal and applies the demodulated signal to the receiver 14. The receiver 14 provides the acoustic voice signal. On the other hand, the electrical signal from the microphone 15 is applied to the modulator 16, which modulates the voice signal to the pulse modulation signal, and inserts that pulse signal in the time slot (y) of FIG. 2. Thus, the power transmission (x), and the voice transmission (y) and (z) are performed on the time divisional basis through a single optical fiber cable.
The important technical matters for facilitating that power transmission through an optical fiber cable are (1) the optical fiber cable with small transmission loss, and (2) the improvement of the conversion efficiency of an optical-electrical converter 11. And it should be noted that those technical matters have been solved at the present stage.
However, the power supply system shown in FIG. 1 has the disadvantage that the power consumption in an exchange station is rather large, since the power supply is always supplied irrespective of the actual status of subscribers. That is to say, even when a subscriber does not operate, the station must supply power for operating an electronic circuit in a subscriber for detecting a calling action of the subscriber. If that power supply were interrupted, the subscriber could not send any signal to the station.
Accordingly, an exchange station must always supply the power pulse to all the subscribers. The electric power necessary for that power supply is estimated as follows. It is assumed that the total of the electrical-optical conversion loss in an exchange station, the transmission loss in an optical fiber cable, and the optical-electrical conversion loss in a subscriber is expected in the range between 10 dB and 20 dB, and the power consumption in electrinic circuits in each subscriber is 0.1 watt. Then, the station must always supply the power supply 1-10 watts to each subscriber. A conventional power supply system through a metallic wire supplies the electric power only when a subscriber is active, and only the voltage is supplied but no power is consumed during a subscriber is inactive. On the contrary, that large power consumption 1-10 watts in case of an optical fiber cable is the serious disadvantage.